During the storage and transit of stacked glass sheets, a common problem is the deterioration in the quality of the surfaces of the glass sheets. In particular, relative movement between adjacent sheets is liable to cause marring of the adjacent glass surfaces. Marring is a particular problem when stacking glass sheets having pyrolytically deposited coatings thereon.
It is generally known in the art to separate adjacent glass surfaces by interposing sheets of paper between the sheets of glass to protect the glass surfaces. However, techniques that utilize paper interleaving are time consuming and costly. Less expensive methods suggested for separating glass sheets involve the use of particulate interleaving materials, which may include natural products such as wood flour and ground corncobs, or synthetic products such as polyethylene, polystyrene or polyacrylate beads, to name a few. Poly(methyl methacrylate) is currently the predominant material used by the glass industry as an interleaving material.
While the synthetic particulate interleaving materials provide a measure of mar protection at relatively low cost, they are sometimes not sufficient to prevent marring on particular glass products, especially pyrolytically coated glass. Pyrolytic coatings of tin oxide, particularly relatively thick ones, are very susceptible to marring, due to their relatively rough surface topography. Such pyrolytic coatings are commonly utilized, for example, in multiple sheet or insulating glass products, and are described further in U.S. Pat. Nos. 4,146,657 and 4,187,336 to Gordon. It would therefore be desirable to provide a method and material for separating stacked glass sheets coated with this material which prevents marring and which is compatible with normal production practices in the glass industry.
As noted above, polyethylene has previously been suggested as a possible interleaving material. For example, U.S. Pat. No. 4,011,359 to Simpkin et al discloses an interleaving material for separating glass sheets and protecting them from scratching and staining. The material comprises a porous, finely divided support material, impregnated with a weakly acidic material, and fine particles of a chemically inert plastic material. The inert plastic separator material may be polyethylene, polystyrene, polytetrafluoroethylene or a methacrylate polyester, and preferably has a larger particle size than the acid-impregnated support material. The interleaving material may be applied to the glass by conventional powder applicators.
U.S. Pat. No. 4,447,496 to Franz et al discloses a method of protecting glass surfaces by the treatment of the glass surfaces with an organotin compound prior to the application of a particulate interleaving material. While various interleaving materials, such as polyethylene, polystyrene, polytetrafluoroethylene and polyacrylate are suggested, Franz et al .indicate that porous cellulose materials such as wood flour are preferred.
U.S. Pat. No. 2,995,533 to Parmer et al discloses a protective coating for glassware which is subjected to abrasive contact with other glassware during handling or shipment. An aqueous emulsion of polyethylene is applied to the glassware. It is noted that the molecular weight of the polyethylene can be as high as 2,000.
While the above materials do provide a measure of protection to normal clear glass, they fail to provide protection to pyrolytic coatings of tin oxide, particularly coatings that are relatively thick, e.g., so called Low-E coatings. It would therefore be desirable to supply a method and material for interleaving both uncoated and pyrolytically coated glass sheets which provides superior resistance to marring during storage or transportation of the glass sheets. In addition, it is desirable for the method and material to prevent the development of low pressures, or even a vacuum, between the stacked glass sheets which may inhibit unloading of the stacked glass sheets.
It must be noted that the prior art referred to hereinabove has been collected and examined only in light of the present invention as a guide. It is not to be inferred that such diverse art would otherwise be assembled absent the motivation provided by the present invention.